Quantum network node and protocols with multiple qubit species

ABSTRACT

The disclosure describes aspects of using multiple species in trapped-ion nodes for quantum networking. In an aspect, a quantum networking node is described that includes multiple memory qubits, each memory qubit being based on a  171 Yb +  atomic ion, and one or more communication qubits, each communication qubit being based on a  138 Ba +  atomic ion. The memory and communication qubits are part of a lattice in an atomic ion trap. In another aspect, a quantum computing system having a modular optical architecture is described that includes multiple quantum networking nodes, each quantum networking node including multiple memory qubits (e.g., based on a  171 Yb +  atomic ion) and one or more communication qubits (e.g., based on a  138 Ba +  atomic ion). The memory and communication qubits are part of a lattice in an atomic ion trap. The system further includes a photonic entangler coupled to each of the multiple quantum networking nodes.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 16/182,219,filed Nov. 6, 2018, now U.S. Pat. No. 10,902,338 and entitled “QUANTUMNETWORK NODE AND PROTOCOLS WITH MULTIPLE QUBIT SPECIES,” which in turnclaims priority to and the benefit of U.S. Provisional PatentApplication No. 62/582,529, entitled “QUANTUM NETWORK NODE AND PROTOCOLSWITH MULTIPLE QUBIT SPECIES,” and filed on Nov. 7, 2017, and also claimspriority to and the benefit of U.S. Provisional Patent Application No.62/694,604, entitled “QUANTUM NETWORK NODE AND PROTOCOLS WITH MULTIPLEQUBIT SPECIES,” filed on Jul. 6, 2018, the contents of each of which areincorporated herein by reference in their entirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under W911NF1520067awarded by the U.S. Army Research Laboratory (ARL) and FA95501610421awarded by the Air Force Office of Scientific Research (AFOSR). Thegovernment has certain rights in the invention.

BACKGROUND OF THE DISCLOSURE

Aspects of the present disclosure generally relate to quantuminformation processing, and more specifically, to techniques for usingmultiple species in a trapped-ion node for quantum networking.

Trapped atomic ions is one of the quantum information processing (QIP)approaches that has delivered universal and fully programmable machines.Trapped atomic ions are also a leading platform for quantum informationnetworks (QINs). Systems or networks based on trapped atomic ions thatcan improve the overall communications of such systems or networks aredesirable.

SUMMARY OF THE DISCLOSURE

The following presents a simplified summary of one or more aspects inorder to provide a basic understanding of such aspects. This summary isnot an extensive overview of all contemplated aspects, and is intendedto neither identify key or critical elements of all aspects nordelineate the scope of any or all aspects. Its purpose is to presentsome concepts of one or more aspects in a simplified form as a preludeto the more detailed description that is presented later.

Trapped atomic ions are a leading platform for QINs, with long-livedidentical qubit memories that can be locally entangled through theirCoulomb interaction and remotely entangled through photonic channels. Toperform both local and remote operations in a single node of a quantumnetwork requires extreme isolation between spectator qubit memories andqubits associated with the photonic interface. This disclosure describeshow to achieve this isolation by co-trapping ¹⁷¹Yb⁺ and ¹³⁸Ba⁺ qubits ina same node. This disclosure further describes the requirements of ascalable ion trap network node based on the results of two distinctexperiments that consist of entangling the mixed species qubit pairthrough their collective motion and entangling a ¹³⁸Ba⁺ qubit with anemitted visible photon.

In an aspect of this disclosure, a quantum networking node for use in amodular optical architecture for quantum computing is described thatincludes multiple memory qubits, each memory qubit being based on a¹⁷¹Yb⁺ atomic ion, and one or more communication qubits, eachcommunication qubit being based on a ¹³⁸Ba⁺ atomic ion. The multiplememory qubits and the one or more communication qubits may be part of alattice in an atomic ion trap.

In another aspect of this disclosure, a quantum computing system havinga modular optical architecture is described that includes multiplequantum networking nodes and a photonic entangler coupled to each of themultiple quantum networking nodes. Each quantum networking node includesmultiple memory qubits, each memory qubit being based on a ¹⁷¹Yb⁺ atomicion, and one or more communication qubits, each communication qubitbeing based on a ¹³⁸Ba⁺ atomic ion. The multiple memory qubits and theone or more communication qubits may be part of a lattice in an atomicion trap.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only some implementation and aretherefore not to be considered limiting of scope.

FIG. 1 is a diagram that illustrates an example of a multi-species iontrap network, in accordance with aspects of this disclosure.

FIGS. 2(a) and 2(b) are diagrams that illustrate examples ofcorrelations between ¹³⁸Ba⁺ qubit and emitted photon polarizations inmultiple bases, in accordance with aspects of this disclosure.

FIG. 3 is a diagram that illustrates an example of off-resonantcouplings of 532 and 355 nm pulsed laser beams to ²P_(1/2) and ²P_(3/2)levels in both ¹³⁸Ba⁺ and ¹⁷¹Yb⁺ in accordance with aspects of thisdisclosure.

FIGS. 4(a) and 4(b) are diagrams that illustrate examples of vibrationalspectrum of a co-trapped ¹³⁸Ba⁺-¹⁷¹Yb⁺ crystal for transvers and axialdirections of motion, in accordance with aspects of this disclosure.

FIGS. 5(a) and 5(b) are diagrams that illustrate examples ofexperimental steps and results of mapping the state of ¹³⁸Ba⁺ to ¹⁷¹Yb⁺using collective motion directly, in accordance with aspects of thisdisclosure.

FIGS. 6(a) and 6(b) are diagrams that illustrate examples of measureprobabilities of the ¹⁷¹Yb and ¹³⁸Ba⁺ qubit states after an entanglingMS gate and π/2 rotation following the MS interaction, in accordancewith aspects of this disclosure.

FIGS. 7(a)-7(c) are diagrams that illustrate examples of network nodes,photonic entanglers, and wavelength converters, in accordance withaspects of this disclosure.

FIG. 8 is a block diagram that illustrates an example of atrapped-ion-based QIP system, in accordance with aspects of thisdisclosure.

FIG. 9 is a diagram that illustrates an example of a computer device, inaccordance with aspects of this disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known components are shown in blockdiagram form in order to avoid obscuring such concepts.

Trapped atomic ions are among the most advanced platforms for quantuminformation networks (QINs), hosting qubit memories that are inherentlyidentical and have unrivaled coherence properties. A single node of thenetwork may be realized with a chain of trapped ions, where localentangling gate operations use external control fields that couple thequbit states through their collective motion. Edges of the network canthen be implemented by photonic entangling operations between select“communication” qubits in separate nodes. However, the photonicinterface for the communication qubits may not disturb the spectatormemory qubits, as even a single resonant photon can destroy the quantummemory. Such isolation is best accomplished by using two differentspecies of atomic ions: one for local processing and memory, the otherfor communicating with other nodes, as shown in FIG. 1.

FIG. 1 shows a diagram 100 with a multispecies ion trap network in which¹³⁸Ba⁺ communication qubits 125 are coupled to optical fibers. Usingphotons entangled with their parent atoms, any pair of ¹³⁸Ba⁺ qubits indifferent nodes 110 a, 110 b, and 110 c, for example, can be entangledthrough a reconfigurable photonic entangler 120. Local Coulombinteractions mediate transfer of this entanglement to nearby ¹⁷¹Yb⁺memory qubits 115 as well as quantum logic gates within the node. Thedisparate or different electronic transition frequencies of the twospecies (e.g., the ¹⁷¹Yb⁺ memory qubits 115 and the ¹³⁸Ba⁺ communicationqubits 125) provides the necessary isolation to protect ¹⁷¹Yb⁺ memoryqubits 115 from resonant processes in the ¹³⁸Ba⁺ photonic interface.

This disclosure describes different ingredients or requirements of amultispecies ion trap node for use in a potential quantum network. Theserequirements can include coherent quantum state mapping between memoryand communication qubits, and the generation of photonic qubitsentangled with the communication qubits. For example, the memory qubitsare encoded in the ²S_(1/2) ground state hyperfine “clock” levels of the¹⁷¹Yb⁺ atomic ions, |F=0, m_(F)=0

≡|

and |F=1, m_(F)=0

≡|

. For the communication qubits, this disclosure proposes the use of the²S_(1/2) ground state electron spin levels of the ¹³⁸Ba⁺ atomic ions,|J=½, m_(J)=−½

≡|↓

and |J=½, m_(J)=+½

≡|↑

. The ¹³⁸Ba⁺ system features relatively long wavelength photon emissionlines (e.g., 493 nanometers (nm) and 650 nm), easing the technologicalrequirements for the photonic interfaces and providing the necessaryisolation from the ¹⁷¹Yb⁺ resonance at 369 nm. The disclosure describesverification of the isolation between these two species by observingthat the measured coherence time of the ¹⁷¹Yb⁺ qubits (˜1.5 seconds) isnot affected by fluorescence or the driving laser light associated witha continuously Doppler-cooled ¹³⁸Ba⁺ qubit that is positioned just a fewmicrons away. With the application of dynamical decoupling pulses, a¹⁷¹Yb⁺ hyperfine qubit coherence time exceeding 10 minutes has beenreported in a similar setup where a nearby ¹³⁸Ba⁺ ion is used forsympathetic cooling.

A standard spin-dependent fluorescence collection may be used for thenear-perfect single-shot detection of the ¹⁷¹Yb⁺ qubit state. The ¹³⁸Ba⁺ion qubit lacks such an isolated cycling transition, therefore, todetect the ¹³⁸Ba⁺ qubit state many identical are averaged. This lack ofan isolated cycling transition is not a problem in the multispeciesnetwork architecture because the ¹³⁸Ba⁺ qubits are being used only as alink between ¹⁷¹Yb⁺ memory qubits. Once the ¹³⁸Ba⁺ qubit is mapped toneighboring ¹⁷¹Yb⁺ memories through Coulomb-based gates, quantuminformation processing need not rely on the state detection of the¹³⁸Ba⁺ communication qubits. Nevertheless, implementing a statemeasurement technique is still useful for a calibration and diagnosticsof the ¹³⁸Ba⁺ system.

In addition to their use as photonic communication qubits, ¹³⁸Ba⁺ ionscan be employed for sympathetic cooling of the ¹⁷¹Yb⁺ qubits to maintainoccupation in low motional phonon eigenstates for higher fidelityquantum operations. An electromagnetically-induced transparency (EIT)cooling technique can be implemented using 493 nm laser beams that aretuned to about 120 MHz blue of the ²S_(1/2)-²P_(1/2) transition. Thelaser beams introduce a narrow atom-laser dressed state resonance wherethe red-sideband transitions are selectively excited, whileblue-sideband and carrier transitions are suppressed. With thistechnique, it may be possible to cool the motion of a ¹³⁸Ba⁺ and ¹⁷¹Yb⁺two ion crystal to n≈0.06 (out-of-phase mode) and n≈0.1 (in-phase mode).

Communication qubits may not require long coherence times, as theinformation can be quickly transferred to memory qubits, where it can bestored and used later. The short coherence time of Zeeman ¹³⁸Ba⁺ qubitsdue to high magnetic field sensitivity of about 2.8 kHz/mG, however,might result in errors during the information transfer operation. It ispossible to use an arbitrary waveform generator to apply a magneticfield at 60 Hz and higher harmonics with full phase and amplitudecontrol to partially cancel the background magnetic field. Thistechnique may increase the ¹³⁸Ba⁺ coherence time from 100 microseconds(μs) to approximately 4 milliseconds (ms), which is much longer than anytransfer operation gate times.

A photonic interface can be demonstrated by entangling the ¹³⁸Ba⁺ qubitwith an emitted photon through a postselection procedure. For example,in about 1 μs, the qubit can be initialized to the |↓

state and weakly excite it to the ²P_(1/2)|J=½, m_(J)=+½

level with probability P_(exc)≈10%. To achieve an average experimentalrepetition rate of ˜500 kHz, 50 μs of Doppler cooling light may beapplied after 50 entanglement attempts. After the excitation, the atomdecays back to the |↓

state emitting a σ⁺-polarized photon, or to the |↓

state emitting a π-polarized photon. The photons perpendicular to thequantization axis are collected; therefore, π photons are registered asvertically polarized in this basis (|V

), while σ⁺ photons are registered as horizontally polarized (|H

). Given that a photon is collected, this ideally results in anentangled state between the ¹³⁸Ba⁺ qubit and the photon polarizationqubit, |↓

|V

+|↑

|H

.

FIGS. 2(a) and 2(b) show the correlations between the ¹³⁸Ba⁺ qubit andthe emitted photon polarization in multiple bases. More particularly,FIGS. 2(a) and 2(b) show correlation measurements between atom andphoton qubit states in multiple bases, and from these measurements itmay be possible to infer the (postselected) entanglement fidelity to be

≥0.86. The errors may be attributed to polarization mixing due to thelarge solid angle (10%), multiple photon scattering in the excitationstep (e.g., P_(exc)/4=2.5%), and imperfect state initialization ordetection (e.g., 1%). These error sources can be significantly reducedby collecting photons along the quantization axis and using pulsedlasers for fast excitation of the atom.

Referring back to FIG. 2(a), a diagram 200 shows the measuredprobability of finding a ¹³⁸Ba⁺ qubit in |↓

conditioned upon detecting photon qubit states |V

(light blue) or |H

(dark red) by using a photo multiplier tube (PMT). A half wave plate(HWP) rotates the photonic qubit, and the data show two measurementscorresponding to HWP angles of 0 and π/4.

Referring back to FIG. 2(b), a diagram 250 shows the photon polarizationbeing rotated by fixing the HWP at π/8 so that |H

→|H

−|V

and |V

→|H

+|V

. Subsequent photon detection projects the atom to a superposition (|↓

+|↑

)|H

+(|↓

−|↑

)|V

. Then, following detection of a |V

or a |H

photon, atomic superposition states are coherently rotated to |↓

and |↑

with a π/2 rotation having a phase of either π/2 or 3π/2, recoveringhigh correlations between the qubit and the photon.

In addition to the aspects described above, this disclosure furtherdescribes a deterministic quantum gate between the two species in aquantum networking node. Coherent Raman transitions can be driven inboth atomic ions (e.g., in both species of atomic ions) using a singlelaser for the coherent exchange of information between the ¹⁷¹Yb⁺ and¹³⁸Ba⁺ qubits. Further described below are a direct Cirac-Zoller (CZ)mapping process by resonantly coupling to the collective motion of thetrapped ions and a dispersive Mølmer-Sørensen (MS) quantum gate betweenthe qubits.

A Nd:YVO4 mode-locked pulsed laser (Spectra-Physics Vanguard) may beused to introduce non-copropagating Raman beams (e.g., beam propagatingin different directions) that can drive transitions between differentvibrational eigenmodes and qubit states. As shown in a diagram 300 inFIG. 3, these beams off-resonantly couple to excited levels, where thefrequency tripled 355 nm output is used for the ¹⁷¹Yb⁺ system, while thefrequency doubled 532 nm output from the same laser is used for the¹³⁸Ba⁺ system. Stimulated Raman transitions are driven when thebeat-note frequency between two beams is near the qubit splitting.Linear polarizations that are all perpendicular to the quantization axismay be chosen, allowing desired Raman transitions to be driven whileminimizing differential ac Stark shifts on each species. The largebandwidth of the frequency comb easily spans the ¹⁷¹Yb⁺ qubit frequencyof 12.642821 GHz for Raman rotations. In order to stabilize thebeat-note frequency of two Raman beams, a feed-forward technique may beused that modulates one of the 355 nm beams to compensate for anychanges of laser repetition rate. Since the ¹³⁸Ba⁺ qubit splitting isonly a few megahertz, multiple comb teeth separations may not be neededfor driving transitions on this qubit, and therefore, beat-notestabilization may not be necessary on 532 nm beams.

Referring back to FIG. 3, there is shown off-resonant couplings of 532and 355 nm pulsed laser beams to ²P_(1/2) and ²P_(3/2) levels in boththe ¹³⁸Ba⁺ and the ¹⁷¹Yb⁺ atomic systems to drive stimulated Ramantransitions, with polarizations indicated. The splittings shown in FIG.3 are provide for purposes of illustration and need not be shown toscale.

While the 355 nm radiation nominally couples only to the ¹⁷¹Yb⁺ qubitand the 532 nm to the ¹³⁸Ba⁺ qubit, there is a small amount ofcross-talk coupling to the other atomic system. For equal intensitiesand without regard to the comb spectrum or the light polarization, the¹⁷¹Yb⁺ system may feel an effective Rabi frequency from the 532 nmradiation that is ˜2.6% of the nominal 355 nm radiation Rabi frequency.Likewise, the ¹³⁸Ba⁺ system may feel an ˜11% Rabi frequency from the 355nm radiation. The required laser polarization and frequency combspectrum, however, are different for the two atomic qubit transitions,and these aspects may be used to reduce cross talk to much less than 1%between the two systems. The spontaneous Raman scattering rate per qubitRabi cycle may be less than 10⁻⁵ for both atomic species, resulting inan error of less than 10⁻⁵ (10⁻⁴) on single (two-)qubit gates. Rarespontaneous scattering in the ¹³⁸Ba⁺ system from 532 nm appears tooptically pump the ¹³⁸Ba⁺ system through the ²P_(3/2) level to themetastable ²D_(5/2) state, which has a lifetime of 32 seconds. Theserare pumping events may be overcome by, for example, illuminating theions with a diffuse 1 Watt, orange light-emitting diode (centered around617 nm) that excites the ²D_(5/2) to ²P_(3/2) transition at 614 nm withenough intensity to return the ion to the ground state in approximately30 milliseconds.

Despite their similar atomic masses, the transverse motion of a coupledpair of ¹³⁸Ba⁺ and ¹⁷¹Yb⁺ ions exhibits a large mismatch in theiramplitude for a given mode, resulting in a smaller motional couplingbetween the ions, as illustrated in a diagram 400 in FIG. 4(a),described in more detail below. For this reason, the better-matchedaxial modes may be used instead, as illustrated in a diagram 450 in FIG.4(b), described in more detail below. As the number of ions in thecrystal chain increases, the motional eigenvector mismatch in thetransverse modes becomes less significant and these modes can be usedconveniently to benefit from higher mode frequencies.

FIGS. 4(a) and 4(b) shows a Raman sideband vibrational spectrum of aco-trapped ¹³⁸Ba⁺−¹⁷¹Yb⁺ crystal for transverse (FIG. 4(a)) and axial(FIG. 4(b)) directions of motion. The measured probability of changingthe qubit state is plotted in light blue for ¹³⁸Ba⁺ and in dark purplefor ¹⁷¹Yb⁺, as a function of detuning from the carrier transition wherethe shared motional phonon state is preserved. The peaks on the positive(negative) values correspond to a blue-(red-) sideband transition inwhich the spin flip is accompanied by the addition (subtraction) of aphonon. The sidebands corresponding to the in-phase (IP) andout-of-phase (OP) are labeled for the transverse (x,y) and axial (z)directions of motion, with their theoretical eigenvector amplitudesindicated at the right. The unlabeled peaks correspond to higher ordersidebands, and interactions involving multiple modes such as subtractionof a phonon from one mode and addition in another.

At first, the qubit state of ¹³⁸Ba⁺ is transferred to ¹⁷¹Yb⁺ by directlyusing the collective motion in a Cirac-Zoller (CZ) mapping scheme. Theprocedure, which is illustrated in a diagram 500 in FIG. 5(a) describedin more detail below, starts with EIT cooling and preparation of the¹³⁸Ba⁺ spin state with a carrier transition. Next, a red-sideband πrotation on the ¹³⁸Ba⁺ system transfers information to a shared phononmode, which is then transferred to the ¹⁷¹Yb⁺ qubit with a furtherred-sideband π rotation on the ¹⁷¹Yb⁺ system. The overall state transferefficiency of 0.75, as shown in a diagram 550 in FIG. 5(b) described inmore detail below, was limited primarily by the purity of the initialmotional state. But the main drawback to the CZ method is the necessityof phase coherence between the communication qubit and the CZ mappingoperations. Because the communication qubit may have prior entanglementthrough the photonic channel, the CZ mapping method requires stabilizingthe beam paths to much better than an optical wavelength.

Referring back to FIG. 5(a), the diagram 500 shows details of theexperimental steps described above for mapping the state of ¹³⁸Ba⁺ to¹⁷¹Yb⁺ using collective motion directly. As noted above, the procedurestarts with EIT cooling (530), followed by the initialization of qubitstates (QI) to |↓

and |

(505, 535). After the initialization, a stimulated Raman rotation R(T)(510) of the ¹³⁸Ba⁺ qubit over time T prepares the state to betransferred. A red-sideband π rotation (RSB π) (515) on the ¹³⁸Ba⁺ qubittransfers this information to a shared phonon mode, which is thentransferred to the ¹⁷¹Yb⁺ qubit with another red-sideband π rotation(520). In a final step, the ¹⁷¹Yb⁺ qubit state may be measured (525).

Referring back to FIG. 5(b), the diagram 550 shows data representing theprobability of finding a ¹⁷¹Yb⁺ qubit in |

as a function of the ¹³⁸Ba⁺ qubit rotation time T, with an observedstate transfer efficiency of ≈0.75.

In addition to the features described above, a Mølmer-Sørensen (MS)transfer method is described that relaxes the above limitations. In theMS transfer scheme, entanglement and state transfer fidelity requireonly confinement to the Lamb-Dicke limit, which may be achieves with 300μs of Doppler cooling followed by 500 μs of EIT cooling. A MS entanglinggate may be realized in the system described herein by simultaneouslyaddressing the axial out-of-phase mode with a symmetric detuning δ usingpairs of non-copropagating Raman beams. Since the pulse pairs of 355 and532 nm follow different paths, they are not necessarily incident on theatoms at the same time. Importantly, a temporal overlap between thesepairs is not necessary for the MS interaction. Spin-dependent forcesusing the Raman beams can be applied at different times to each atom.The outcome is just a static phase on the entangled state which can becontrolled by adjusting either the optical path lengths or thedifference between the radio frequency (RF or rf) beatnote phases of the355 and 532 nm driving fields. These spin-dependent forces displace themotional wave packets of certain two-qubit states in phase space. Walshmodulation may be incorporated to suppress frequency and timing errors,and, after a gate time T=200 μs with a detuning of δ=10 kHz, the motionreturns to its original state, picking up a geometrical phase as in theusual MS gate. The optical intensities of the driving fields areadjusted to obtain carrier Rabi frequencies of Ω=δ/4η to result in a π/2geometrical phase after the MS interaction, where η is the Lamb-Dickeparameter. We find the correct optical force phase by monitoring theacquired geometrical phases. To maintain a shot-to-shot relative opticalforce phase, the same arbitrary waveform generator may be used to driveacousto-optic modulators for the 355 and 532 nm laser beams. Thefidelity of this operation is approximately

=0.60, as shown in FIGS. 6(a) and 6(b), and this low fidelity may beattributed to, for example, excessive heating ({dot over (n)}≈5 ms⁻¹) ofthe axial out-of-phase mode.

FIG. 6(a) shows a diagram 600 that illustrates measured probabilities ofthe ¹⁷¹Yb⁺ and ¹³⁸Ba⁺ qubit states after an entangling MS gate isapplied to the initial |

↓

state. This interaction would ideally create the maximally entangled(1/√{square root over (2)})(|

↓

−e^(−ϕ) ^(s) |

↑

) state. The optical phases of the driving fields are imprinted on spinswith the gate phase, ϕ_(s).

FIG. 6(b) shows a diagram 650 following the MS interaction, where π/2rotations are applied to both qubits. The phase of the ¹³⁸Ba⁺π/2 pulseis kept constant, while the ¹⁷¹Yb⁺ π/2 rotation phase is scanned. Thedata shows measured qubit state probabilities at the maximum paritypoints |P(|

↓

)+P(|

↑

)−P(|

↑

)−P(|

↓

)|.

Even though the phases of the optical fields are imprinted on theentangled state after this interaction, two consecutive MS gates with arelative π phase difference can be used to coherently transfer theinformation from communication qubit (e.g., ¹³⁸Ba⁺ qubit) to memoryqubit (e.g., ¹⁷¹Yb⁺ qubit) without imprinting an extra optical phase.Thus, phase coherence between remote and local entanglement operationsin the quantum network can be established without a need for directlyeliminating optical phase dependence from the MS gate with extra singlequbit operations or special beam geometries.

Based on the various techniques and aspects described in thisdisclosure, it is possible to extend a quantum network to many nodesusing photonic Bell state analyzers to make the photonic connections.Considerable improvements on the atom-photon and atom-atom entanglementfidelities and rates are possible in order to scale to manyinterconnected nodes. First, the encoding of photonic qubits into twodifferent frequencies rather than into polarization is expected toprovide significant improvements in the remote communication qubitfidelity. Second, the use of fabricated chip traps with integratedoptical elements is expected to enhance the connection rate betweennodes. Additionally, the positional stability of the ions stemming fromthe uniformity and repeatability of construction, as well as heatingrates comparable to hand assembled traps (such as the Sandia NationalLaboratories high-optical-access microfabricated ion trap with {dot over(n)}≈40 s⁻¹), would likely allow for much higher fidelity motional gatesbetween memory and communication qubits in these fabricated traps.

Additional aspects related to the multispecies ion trap networkdescribed above are presented below. For example, a quantum networkingnode (e.g., nodes 110 a, 110 b, and 110 c in FIG. 1) for use in amodular optical architecture for quantum computing can include multiplememory qubits (e.g., memory qubits 115), each memory qubit being basedon a ¹⁷¹Yb⁺ atomic ion, and one or more communication qubits (e.g.,communication qubits 125), each communication qubit being based on a¹³⁸Ba⁺ atomic ion. The multiple memory qubits and the one or morecommunication qubits may be part of a lattice in an atomic ion trap (seee.g., ion trap 870 in FIG. 8). In an example, the lattice can be alinear lattice.

In an aspect of such a quantum networking node, a localized connectionbetween the ¹⁷¹Yb⁺ atomic ion of one of the memory qubits and the ¹³⁸Ba⁺atomic ion of one of the communication qubits is made through couplingof the one memory qubit and the one communication qubit by at leastpartially controlling their collective motion through external laserfields configured to apply forces to the ¹⁷¹Yb⁺ atomic ion of the onememory qubit, the ¹³⁸Ba⁺ atomic ion of the one communication qubit, orboth. For example, the atomic ion trap can be configured to receive asingle laser, a second harmonic of the single laser corresponds to afirst external laser field that applies forces to the ¹³⁸Ba⁺ atomic ionof the one communication qubit, and a third harmonic of the single lasercorresponds to a second external laser field that applies forces to the¹⁷¹Yb⁺ atomic ion of the one memory qubit. Moreover, the second harmonicof the single laser is at an emission line of a wavelength ofapproximately 532 nm, and wherein the third harmonic of the single laseris at an emission line of a wavelength of approximately 355 nm.

In another aspect of such a quantum networking node, the ¹⁷¹Yb⁺ atomicion is a first atomic species, the ¹³⁸Ba⁺ atomic ion is a second atomicspecies, and an atomic mass of the first atomic species and an atomicmass of the second atomic species are substantially similar, with anatomic mass difference between the two atomic masses of less than 25%.

In another aspect of such a quantum networking node, the one or morecommunication qubits include multiple communication qubits, and thequantum networking node is configured to multiplex between the multiplecommunication qubits to enable repeated trials of photon emission fromthe multiple communication qubits. Moreover, the one or morecommunication qubits include multiple communication qubits positioned atone end of the lattice, at another end of the lattice, or at both endsof the lattice. For example, FIG. 7(a) shows diagrams 700, 710, and 720that illustrate examples of a node 110 having multiple communicationqubits 125. The diagram 700 shows a node 110 with multiple communicationqubits (e.g., communication bits 125), where at least one communicationqubits is positioned at each end of a lattice (which also includesmemory qubits 115). The diagram 710 shows a node 110 with multiplecommunication qubits (e.g., communication qubits 125) at one end of alattice (which also includes memory qubits 115). The diagram 720 shows anode 110 with multiple communication qubits (e.g., communication qubits125) at another end of a lattice (which also includes memory qubits115).

In another aspect of such a quantum networking node, the ¹³⁸Ba⁺ atomicion of any one of the one or more communication qubits is configured toemit a photon through fluorescence, the emitted photon being entangledwith the ¹³⁸Ba⁺ atomic ion. In an example, at least a portion of aspectrum of the emitted photon is in the visible spectrum. The emissionlines of the emitted photon can be isolated from a resonance of the¹⁷¹Yb⁺ atomic ions of the multiple memory qubits. The emission lines caninclude emission lines at wavelengths of approximately 493 nm and 650nm, and wherein the resonance of the ¹⁷¹Yb⁺ atomic ions is at awavelength of approximately 369 nm. The emission lines at wavelengths ofapproximately 493 nm and 650 nm can correspond to visible optical linesconnecting electronic ground-level Zeeman qubit states of the ¹³⁸Ba⁺atomic ion to excited states.

In another aspect of such a quantum networking node, each memory qubitcan be configured to be encoded in the ²S_(1/2) ground state hyperfinelevel of the respective ¹⁷¹Yb⁺ atomic ion. A hyperfine state coherencetime of the respective ¹⁷¹Yb⁺ atomic ion is approximately 1.5 seconds orgreater. The splitting of hyperfine qubit states of the respective¹⁷¹Yb⁺ atomic ion is highly insensitive to magnetic field fluctuations(much less than the electron magnetic moment or “Bohr Magneton” of 1.4MHz/Gauss). Moreover, each memory qubit can be further configured forinitialization and detection without having to shuttle atomic statepopulation between hyperfine levels.

In another aspect of such a quantum networking node, each communicationqubit can be configured to use the ²S_(1/2) ground state electron spinlevels of the ¹³⁸Ba⁺ atomic ion.

Another example related to the multispecies ion trap network describedabove can be a quantum computing system having a modular opticalarchitecture that can include multiple quantum networking nodes (e.g.,nodes 110 a, 110 b, and 110 c in FIG. 1), where each quantum networkingnode includes multiple memory qubits (e.g., memory qubits 115), eachmemory qubit being based on a ¹⁷¹Yb⁺ atomic ion, and one or morecommunication qubits (e.g., communication qubits 125), eachcommunication qubit being based on a ¹³⁸Ba⁺ atomic ion. The multiplememory qubits and the one or more communication qubits can be part of alattice in an atomic ion trap (see e.g., ion trap 870 in FIG. 8). Theone or more communication qubits in each of the multiple quantumnetworking nodes include multiple communication qubits positioned at oneend of the lattice, at another end of the lattice, or at both ends ofthe lattice (see e.g., FIG. 7(a)). The system can further include aphotonic entangler (e.g., photonic entangler 120 in FIG. 1) coupled toeach of the multiple quantum networking nodes.

In an aspect of such a quantum computing system, each memory qubit ineach of the multiple quantum networking nodes is configured to beencoded in the ²S_(1/2) ground state hyperfine level of the respective¹⁷¹Yb⁺ atomic ion.

In another aspect of such a quantum computing system, each communicationqubit in each of the multiple quantum networking nodes is configured touse the ²S_(1/2) ground state electron spin levels of the ¹³⁸Ba⁺ atomicion.

In another aspect of such a quantum computing system, a firstcommunication qubit of the one or more communication qubits of a firstquantum networking node of the multiple quantum networking nodes isentangled with one of the one or more communication qubits in a secondquantum networking node of the multiple quantum networking nodes.Moreover, a second communication qubit of the one or more communicationqubits of the first quantum networking node is entangled with one of theone or more communication qubits in a third quantum networking node ofthe multiple quantum networking nodes.

In another aspect of such a quantum computing system, the ¹³⁸Ba⁺ atomicion of any one of the one or more communication qubits in one of themultiple quantum networking nodes is configured to emit a photon throughfluorescence, the emitted photon being entangled with the ¹³⁸Ba⁺ atomicion. At least a portion of a spectrum of the emitted photon is in thevisible spectrum. Moreover, the photonic entangler is coupled to each ofthe multiple quantum networking nodes via optical fibers configured tooperate at an optical communications spectrum, the quantum computingsystem further comprising a wavelength converter to convert the visiblespectrum of the emitted photon to the optical communications spectrum.In an example, the optical communications spectrum includes wavelengthsof approximately 1300-1550 nm. The photonic entangler can include one ormore optical components compatible with the optical communicationsspectrum. For example, the photonic entangler can include one or morereconfigurable optical switches to enable entanglement between any twoquantum networking nodes from the multiple quantum networking nodes.FIGS. 7(b) and 7(c) respectively illustrate examples of a photonicentangler and a wavelength converter. For example, a diagram 730 in FIG.7(b) shows the photonic entangler 120 having one or more opticalcomponents 735 a, . . . , 735 n, where these components can include, asdescribed above, components compatible with the optical communicationsspectrum such as reconfigurable optical switches. A diagram 750 in FIG.7(c) shows a wavelength converter 760 configured to convert light and/orphotons carrying some form of information from the visible spectrum tothe optical communications spectrum.

While the various examples of memory qubits 115 and communication qubits125 have been described above in relation to ¹⁷¹Yb⁺ memory qubits 115and the ¹³⁸Ba⁺ communication qubits 125, respectively, it is to beunderstood that these examples are provided for purposes of illustrationand not of limitation. The memory qubits 115 and the communicationqubits 125 can be selected to be sufficiently different so that thecommunication qubits 125 do not affect the functionality of the memoryqubits 115. This may be achieved by having the memory qubits 115 and thecommunication qubits 125 be (1) made of different ion species, (2) madeof different isotopes of the same species, as long as there aresufficient differences, (3) choice of different qubit basis states(e.g., hyperfine states versus other types of states), or (4) acombination of the above.

For example, with respect to different ion species, Yb/Ba can be used asdescribed above, but also Ca/Sr, Be/Mg, or a combination of any ofthese. Others may also include Cd, Zn, Al, etc. For these species,different isotopes may be used. For example, for Yb/Ba, any of 171Yb,174Yb, 176Yb, or 172Yb, etc. and any of 138Ba, 137Ba, 133Ba, etc. can beused. For Ca/Sr, any of 40Ca, 43Ca, etc. and any of 88Sr, 87Sr, 86Sr,etc. can be used. For Be/Mg, 9Be and any of 24Mg, 25 Mg, 26Mg, etc. canbe used.

The choice of each qubit when using the different ion species describedabove can be hyperfine (odd isotopes of each ion species), Zeeman (anyof the species), or optical qubit (for Yb, Ba, Ca, Sr, and Mg, forexample).

Also as described above, different isotopes of the same species can beused for memory qubits 115 and communication qubits 125. For example,two of 171Yb, 174Yb, 176Yb, or 172Yb, etc. can be used for memory andcommunication qubits, two of 138Ba, 137Ba, 133Ba, etc. can be used formemory and communication qubits, two of 40Ca, 43Ca, etc. can be used formemory and communication qubits, two of 88Sr, 87Sr, 86Sr, etc. can beused for memory and communication qubits, or two of 24Mg, 25 Mg, 26Mg,etc. can be used for memory and communication qubits.

Moreover, different nodes 110 can be made using different combinationsof memory qubits 115 and communication qubits 125. For example, one node110 can be made using one set of ion species (e.g., Yb/Ba) and anothernode 110 can be made using another set of ion species (e.g., Ca/Sr orBe/Mg). In another example, one node 110 can be made using one set ofisotopes of the same species (e.g., different isotopes of Yb) andanother node 110 can be made using another set of isotopes of the samespecies (e.g., different isotopes of Ca). Different combinations ofnodes made of different sets of ion species, different sets of isotopesof the same species, and/or different qubit choice of qubit basis states(e.g., hyperfine) can be used as part of a multispecies ion trap networksuch as the one described above in the diagram 100 in FIG. 1

FIG. 8 is a block diagram that illustrates an example of a QIP system800 in accordance with aspects of this disclosure. The QIP system 800may also be referred to as a quantum computing system, a quantumcomputing network, a computer device, or the like. In an aspect, the QIPsystem 800 may correspond to portions of the multispecies ion trapnetwork in FIG. 1 or a quantum computer implementation of the computingdevice 900 in FIG. 9.

The QIP system 800 can include a source 860 that provides atomic speciesto a chamber 850 having an ion trap 870 that traps the atomic speciesonce ionized by an optical controller 820. Optical sources 830 in theoptical controller 820 may include one or more laser sources that can beused for ionization of the atomic species, control (e.g., phase control)of the atomic ions, for fluorescence of the atomic ions that can bemonitored and tracked by image processing algorithms operating in animaging system 1040 in the optical controller 1020, and/or for otheraspects including those described above in connection with usingmultiple species in a trapped-ion node for quantum networking. Theimaging system 840 can include a high resolution imager (e.g., CCDcamera) for monitoring the atomic ions while they are being provided tothe ion trap 870 (e.g., for counting) or after they have been providedto the ion trap 870 (e.g., for monitoring the atomic ions states). In anaspect, the imaging system 840 can be implemented separate from theoptical controller 820, however, the use of fluorescence to detect,identify, and label atomic ions using image processing algorithms mayneed to be coordinated with the optical controller 820.

The QIP system 800 may also include an algorithms component 810 that mayoperate with other parts of the QIP system 800 (not shown) to performquantum algorithms (e.g., QFT, quantum simulations) that make use of theimplementations described above. The algorithms component 810 mayprovide instructions to various components of the QIP system 800 (e.g.,to the optical controller 1020) to enable the implementation of quantumcircuits, or their equivalents. That is, the algorithms component 810may allow for mapping of different computing primitives into physicalrepresentations using, for example, the ion trap 870.

The QIP system 800 can implement one or more of the components orstructures shown in the multispecies ion trap network in the diagram 100in FIG. 1, as well as some or all or the components shown in FIGS. 7(a),7(b), and 7(c).

Referring now to FIG. 9, illustrated is an example computer device 900in accordance with aspects of the disclosure. The computer device 900can represent a single computing device, multiple computing devices, adistributed computing system, or at least a portion of a computingnetwork, for example. The computer device 900 may be configured as aquantum computer, a classical computer, or a combination of quantum andclassical computing functions.

In one example, the computer device 900 may include a processor 910 forcarrying out processing functions associated with one or more of thefeatures described herein. The processor 910 may include a single ormultiple set of processors or multi-core processors. Moreover, theprocessor 910 may be implemented as an integrated processing systemand/or a distributed processing system. The processor 910 may include acentral processing unit (CPU), a quantum processing unit (QPU), agraphical processing unit (GPU), or combination of those types ofprocessors.

In an example, the computer device 900 may include a memory 920 forstoring instructions executable by the processor 910 for carrying outthe functions described herein. In an implementation, for example, thememory 920 may correspond to a computer-readable storage medium thatstores code or instructions to perform one or more of the functions oroperations described herein. In one example, the memory 920 may includeone or more memory qubits.

Further, the computer device 900 may include a communications component930 that provides for establishing and maintaining communications withone or more parties utilizing hardware, software, and services asdescribed herein. The communications component 930 may carrycommunications between components on the computer device 900, as well asbetween the computer device 900 and external devices, such as deviceslocated across a communications network and/or devices serially orlocally connected to computer device 900. For example, thecommunications component 930 may include one or more buses, and mayfurther include transmit chain components and receive chain componentsassociated with a transmitter and receiver, respectively, operable forinterfacing with external devices. Aspects of the communicationscomponent 930 may be used to implement the multispecies ion trap networkshown in FIG. 1.

Additionally, the computer device 900 may include a data store 940,which can be any suitable combination of hardware and/or software, thatprovides for mass storage of information, databases, and programsemployed in connection with implementations described herein. Forexample, the data store 940 may be a data repository for operatingsystem 960 (e.g., classical OS, or quantum OS). In one implementation,the data store 940 may include the memory 920.

The computer device 900 may also include a user interface component 950operable to receive inputs from a user of the computer device 900 andfurther operable to generate outputs for presentation to the user or toprovide to a different system (directly or indirectly). The userinterface component 950 may include one or more input devices, includingbut not limited to a keyboard, a number pad, a mouse, a touch-sensitivedisplay, a digitizer, a navigation key, a function key, a microphone, avoice recognition component, any other mechanism capable of receiving aninput from a user, or any combination thereof. Further, the userinterface component 950 may include one or more output devices,including but not limited to a display, a speaker, a haptic feedbackmechanism, a printer, any other mechanism capable of presenting anoutput to a user, or any combination thereof.

In an implementation, the user interface component 950 may transmitand/or receive messages corresponding to the operation of the operatingsystem 960. In addition, the processor 910 may execute the operatingsystem 960 and/or applications or programs, and the memory 920 or thedata store 940 may store them.

When the computer device 900 is implemented as part of a cloud-basedinfrastructure solution, the user interface component 950 may be used toallow a user of the cloud-based infrastructure solution to remotelyinteract with the computer device 900.

The computer device 900 can implement one or more of the components orstructures shown in the multispecies ion trap network in the diagram 100in FIG. 1, as well as some or all or the components shown in FIGS. 7(a),7(b), and 7(c). For example, the communications component 930 canimplement one or more of the nodes 110, the photonic entangler 120,and/or the wavelength converter 760.

Although the present disclosure has been provided in accordance with theimplementations shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the scope of the present disclosure.Accordingly, many modifications may be made by one of ordinary skill inthe art without departing from the scope of the appended claims.

What is claimed is:
 1. A quantum computing system having a modularoptical architecture, comprising: one or more first quantum networkingnodes, each of the first quantum networking nodes including: multiplememory qubits, and one or more communication qubits, the multiple memoryqubits and the one or more communication qubits in each of the firstquantum networking nodes being made from different isotopes of a samespecies; one or more second quantum networking nodes, where each of thesecond quantum networking nodes includes: multiple memory qubits, andone or more communication qubits, the multiple memory qubits and the oneor more communication qubits in each of the second quantum networkingnodes being made from different species; and a photonic entangleroptically coupled to each of the one or more first quantum networkingnodes and each of the one or more second quantum networking nodes. 2.The quantum computing system of claim 1, wherein the different isotopesof the same species include different Ba isotopes.
 3. The quantumcomputing system of claim 2, wherein the different Ba isotopes are frommultiple Ba isotopes including at least 138Ba, 137Ba, and 133Ba.
 4. Thequantum computing system of claim 1, wherein the different speciesinclude Yb as one species and Ba as another species.
 5. The quantumcomputing system of claim 1, wherein each of the first quantumnetworking nodes includes an ion trap, and the multiple memory qubitsand the one or more communication qubits of each of the first quantumnetworking nodes are ions trapped in the corresponding ion trap.
 6. Thequantum computing system of claim 1, wherein each of the second quantumnetworking nodes includes an ion trap, and the multiple memory qubitsand the one or more communication qubits of each of the second quantumnetworking nodes are ions trapped in the corresponding ion trap.
 7. Thequantum computing system of claim 1, wherein one of the one or morecommunication qubits of one of the first quantum networking nodes isentangled with one of the one or more communication qubits of one of thesecond quantum networking nodes.
 8. The quantum computing system ofclaim 1, wherein one of the one or more communication qubits of one ofthe first quantum networking nodes is entangled with one of the one ormore communication qubits of another one of the first quantum networkingnodes.
 9. The quantum computing system of claim 1, wherein one of theone or more communication qubits of one of the second quantum networkingnodes is entangled with one of the one or more communication qubits ofanother one of the second quantum networking nodes.
 10. The quantumcomputing system of claim 1, wherein the photonic entangler is opticallycoupled to each of the one or more first quantum networking nodes andeach of the one or more second quantum networking nodes via opticalfibers configured to operate at an optical communications spectrum. 11.A quantum computing system having a modular optical architecture,comprising: one or more first quantum networking nodes, each of thefirst quantum networking nodes including: multiple memory qubits, andone or more communication qubits, the multiple memory qubits and the oneor more communication qubits in each of the first quantum networkingnodes being made from a first set of different species; one or moresecond quantum networking nodes, where each of the second quantumnetworking nodes includes: multiple memory qubits, and one or morecommunication qubits, the multiple memory qubits and the one or morecommunication qubits in each of the second quantum networking nodesbeing made from a second set of different species different from thefirst set of different species; and a photonic entangler opticallycoupled to each of the one or more first quantum networking nodes andeach of the one or more second quantum networking nodes.
 12. The quantumcomputing system of claim 11, wherein the first set of different speciesincludes Yb as one species and Ba as another species.
 13. The quantumcomputing system of claim 11, wherein each of the first quantumnetworking nodes includes an ion trap, and the multiple memory qubitsand the one or more communication qubits of each of the first quantumnetworking nodes are ions trapped in the corresponding ion trap.
 14. Thequantum computing system of claim 11, wherein each of the second quantumnetworking nodes includes an ion trap, and the multiple memory qubitsand the one or more communication qubits of each of the second quantumnetworking nodes are ions trapped in the corresponding ion trap.
 15. Thequantum computing system of claim 11, wherein one of the one or morecommunication qubits of one of the first quantum networking nodes isentangled with one of the one or more communication qubits of one of thesecond quantum networking nodes.
 16. A quantum computing system having amodular optical architecture, comprising: one or more first quantumnetworking nodes, each of the first quantum networking nodes including:multiple memory qubits, and one or more communication qubits, themultiple memory qubits and the one or more communication qubits in eachof the first quantum networking nodes being made from different isotopesof a same first species; one or more second quantum networking nodes,where each of the second quantum networking nodes includes: multiplememory qubits, and one or more communication qubits, the multiple memoryqubits and the one or more communication qubits in each of the secondquantum networking nodes being made from different isotopes of a samesecond species; and a photonic entangler optically coupled to each ofthe one or more first quantum networking nodes and each of the one ormore second quantum networking nodes.
 17. The quantum computing systemof claim 16, wherein the different isotopes of the same first speciesincludes a pair of different Ba isotopes, the pair of different Baisotopes being from multiple Ba isotopes including at least 138Ba,137Ba, and 133Ba.
 18. The quantum computing system of claim 16, whereineach of the first quantum networking nodes includes an ion trap, and themultiple memory qubits and the one or more communication qubits of eachof the first quantum networking nodes are ions trapped in thecorresponding ion trap.
 19. The quantum computing system of claim 16,wherein each of the second quantum networking nodes includes an iontrap, and the multiple memory qubits and the one or more communicationqubits of each of the second quantum networking nodes are ions trappedin the corresponding ion trap.
 20. The quantum computing system of claim16, wherein one of the one or more communication qubits of one of thefirst quantum networking nodes is entangled with one of the one or morecommunication qubits of one of the second quantum networking nodes.